SwiftText

SwiftText lives in your Mac's menu bar. Just click the icon when you need to make a note and SwiftText's window will appear.

No need to save your scratchpad; SwiftText saves it a few seconds after each change you make.

We'll let SwiftText speak for itself:

SwiftText is a quick-access scratchpad for your Mac. Configure SwiftText with a keyboard shortcut so you can get to your scratchpad in the blink of an eye. SwiftText disappears with a keystroke when you're done so you can get back to the task at hand.